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Thermal Analyses of Sodalite, Tugtupite, Danalite and Helvite

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Thermal Analyses of Sodalite, Tugtupite, Danalite and Helvite 163 The Canadian Mineralogist Vol. 40, pp. 163-172 (2002) THERMAL ANALYSES OF SODALITE, TUGTUPITE, DANALITE AND HELVITE SYTLE M. ANTAO Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, U.S.A. ISHMAEL HASSAN§ Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica ABSTRACT Differential thermal analyses (DTA) and thermogravimetric analyses (TG) were carried out on sodalite, tugtupite, danalite, and helvite using a Netzsch STA 409 EP/3/D simultaneous TG–DTA apparatus. In the DTA and TG experimental run, a weighed amount of finely powdered sample was heated from 20 to 1450°C at a constant rate of 5°C/min. Sodalite, Na8[Al6Si6O24]Cl2, melts at 1079°C. The NaCl component in the sodalite-derived melt is lost in two stages; about 4.5 wt.% of NaCl is lost slowly at about 1150°C, and about 7.0 wt.% of NaCl is lost at a faster rate at about 1284°C. The interpretation that the NaCl component behaves this way in sodalite is confirmed by DTA and TG analyses of halite. Tugtupite, Na8[Al2Be2Si8O24]Cl2, melts at 1029°C. The NaCl component in tugtupite is lost in two main stages; about 1.8 wt.% of NaCl is first lost at about 1007°C, and about 8.2 wt.% of NaCl is lost in several steps between about 1018 and about 1442°C. Danalite, ideally Fe8[Be6Si6O24]S2, and helvite, 2+ 3+ ideally Mn8[Be6Si6O24]S2, undergo an oxidation of (Mn, Fe) to (Mn, Fe) cations. This is followed by a loss of S2(g), and then melting at about 1060°C. Finally, there is another oxidation of Mn3+ to Mn4+. These oxidations occur because the samples were heated in a static air environment. The first oxidation in danalite and helvite begins at about 771 and 705°C, respectively. Danalite gains 4.0 wt.%, whereas helvite gains 5.0 wt.% in the first weight-gain stage. In the weight-loss stage, danalite loses 5.7 wt.% of S2(g) from about 1029°C, whereas helvite loses 4.7 wt.% of S2(g) from about 883°C. The second stage of oxidation begins at about 1300°C in both the danalite- and helvite-derived melts. At this stage, the weight gain in the danalite-derived melt is about 0.7%, whereas that in the helvite-derived melt is about 0.2%. Keywords: sodalite, tugtupite, danalite, helvite, differential thermal analysis, thermogravimetric analysis. SOMMAIRE Nous avons étudié la sodalite, la tugtupite, la danalite et la helvite par analyses thermique différentielle (ATD) et thermogravimétrique (TG) en utilisant un appareil Netzsch STA 409 EP/3/D. Pour ces expériences, nous nous sommes servis d’une quantité connue d’échantillon finement pulvérisé, chauffé à partir de 20 jusqu’à 1450°C à un taux constant de 5°C/min. La sodalite, Na8[Al6Si6O24]Cl2, atteint son point de fusion à 1079°C. La composante NaCl du bain fondu se volatilise en deux étapes; environ 4.5% (poids) de NaCl est perdu lentement près de 1150°C, et environ 7.0% est perdu plus rapidement près de 1284°C. Notre interpretation à propos de la sodalite est confirmée par analyses ADT et TG de la halite. La tugtupite, Na8[Al2Be2Si8O24]Cl2, fond à 1029°C. La composante NaCl de la tugtupite se volatilise en deux étapes; environ 1.8% de NaCl se perd près de 1007°C, et environ 8.2% de NaCl se perd en plusieurs étapes entre 1018 et 1442°C. La danalite, dont la composition idéale est 2+ 3+ Fe8[Be6Si6O24]S2, et la helvite, Mn8[Be6Si6O24]S2, subissent une oxydation de (Mn, Fe) à (Mn, Fe) . Cette transformation est 3+ 4+ suivie d’une perte de S2(g), et de la fusion à environ 1060°C. Enfin, il y a une oxydation plus poussée de Mn à Mn . Ces étapes d’oxydation sont le résultat du chauffage des échantillons dans l’air, dans un milieu statique. La première étape d’oxydation de la danalite et de la helvite débute à environ 771 et 705°C, respectivement. La danalite augmente en poids de 4.0%, tandis que la helvite augmente de 5.0%. Suit ensuite une perte de poids, 5.7% sous forme de S2(g) pour la danalite à partir de 1029°C, et 4.7% de S2(g) à partir de 883°C pour la helvite. Le second stade d’oxydation débute à environ 1300°C dans les liquides dérivés de la danalite et de la helvite. L’augmentation en poids à ce stade est d’environ 0.7% pour le cas de la danalite et 0.2% pour le cas de la helvite. (Traduit par la Rédaction) Mots-clés: sodalite, tugtupite, danalite, helvite, analyse thermique différentielle, analyse thermogravimétrique. § E-mail address: [email protected] 163 40#1-fév-02-2328-11 163 3/28/02, 16:26 164 THE CANADIAN MINERALOGIST INTRODUCTION measure the temperature where changes take place, and to compare the results for sodalite and tugtupite. If NaCl Our aim in this study is to determine the thermal is liberated, then it would be of interest to compare the behavior of sodalite, tugtupite, danalite, and helvite us- results for sodalite and tugtupite with those for halite, ing differential thermal analysis (DTA) and thermogra- NaCl. For this purpose, halite also was analyzed by the vimetry (TG) techniques. The types of volatiles liberated differential thermal analysis (DTA) and thermogravi- and the temperatures at which liberation occurs are de- metric analysis (TG) techniques. termined, among other features. From the chemical composition of danalite and In sodalite and tugtupite, Cl and possibly Na are helvite, S is an important volatile that could be liberated important volatile constituents that may be liberated on on heating. Therefore, this study on danalite and helvite heating. If the formula of sodalite is written as was carried out to determine the temperature where Na8[Al6Si6O24]Cl2, then Cl2 (g) could be expected to be S2(g) is liberated, and to determine the temperatures at liberated on heating; however, if the sodalite formula is which other structural changes may occur. written as Na6[Al6Si6O24]•2NaCl, then NaCl could be liberated on heating; similarly for tugtupite, Na8[Al2Be2 REVIEW OF THE STRUCTURAL ASPECTS Si8O24]Cl2, which can be written as Na6[Al2Be2Si8 O24]•2NaCl. This study was carried out to determine The frameworks of tugtupite, danalite, helvite, which chemical constituents are liberated on heating, to lazurite, nosean, and haüyne, which has the sodalite 4+ 3+ structure, consist of TO4 tetrahedra (T = Si , Al , and Be2+), which are linked at the corners and give rise to large cages (Fig. 1). These tetrahedra are fully ordered. The aluminosilicate framework of cubic sodalite, ide- ally Na8[Al6Si6O24]Cl2, is characterized by parallel six- membered rings consisting of alternating AlO4 and SiO4 tetrahedra. The cubic symmetry is the result of the stack- ing of such six-membered rings in an ABCABC... se- quence, which leads to large cages that are bounded by eight six-membered rings parallel to {111} planes and six four-membered rings parallel to {001} planes. The four-membered rings also consist of alternating AlO4 and SiO4 tetrahedra (Fig. 1a). The sodalite or ␤-cages 3+ enclose [Na4•Cl] clusters (Figs. 1b, c). The structure of tugtupite (tetragonal), ideally Na8[Al2Be2Si8O24]Cl2, is isotypic with that of sodalite. The ␤-cages in tugtupite 3+ also enclose [Na4•Cl] clusters, as in sodalite (Hassan & Grundy 1991). The sodalite-group minerals were studied by several researchers (e.g., Pauling 1930, Löns & Schulz 1967, Taylor 1967, 1968, Danø 1966, Peterson 1983, Hassan & Grundy 1984, 1991). The helvite-group minerals, (Mn,Fe,Zn)8[Be6Si6 O24]S2, are also isotypic with sodalite. The Mn-rich member is helvite, the Fe-rich member is danalite, and the Zn-rich member is genthelvite. These minerals were also studied by several researchers (e.g., Hassan & Grundy 1985, Barth 1926, Burt 1980, Dunn 1976). The ␤-cages in the helvite-group minerals enclose clusters 6+ of [(Mn,Fe,Zn)4•S] . Because these clusters are highly charged (i.e., 6+ valence units), they have smaller sizes 3+ than the [Na4•Cl] clusters of sodalite and tugtupite (Hassan & Grundy 1985, 1991). The dimensions of the framework tetrahedra in the helvite-group minerals are constant regardless of the nature of the interframework FIG. 1. (a) Projection of the unit cell of the sodalite structure cation because of the rotational freedom of the frame- down [001], drawn using the program STRUPLO. (b) The work tetrahedra (Hassan & Grundy 1985). In particu- 6+ truncated cubo-octahedral or ␤-cage in sodalite. (c) lar, pure danalite consists of 100% [Fe4•S] clusters, 6+ Fourfold (trigonal pyramid) coordination of the Na site in and pure helvite consists of 100% [Mn4•S] clusters. sodalite. These two minerals are investigated in this study. 163 40#1-fév-02-2328-11 164 3/28/02, 16:26 THERMAL ANALYSES OF SODALITE, TUGTUPITE, DANALITE AND HELVITE 165 EXPERIMENTAL position is given in Table 1. For the thermal analyses, 98.7 mg of finely powdered sodalite sample was used. The samples of sodalite, tugtupite, danalite, and The TG and DTA curves and their corresponding de- helvite were crushed, and pure crystals were handpicked rivative curves (DTG and DDTA, respectively) for so- and crushed to a powder using an agate mortar and dalite are shown in Figure 2. The temperature range pestle. A portion of the powder was used for the DTA from 20 to 850°C did not lead to any significant change and TG analyses, and another portion was used for high- in weight or peaks, and is therefore left out of Figure 2.
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